Microcomputers are sophisticated, general purpose logic devices which can be programmed to perform a wide variety of useful control functions in industrial and communications equipment, large-scale and medium-scale computer peripheral and terminal hardware, automobiles and other transportation media, amusement and educational devices, and the like.
In a microcomputer having on-board serial I/O data communication logic, and in fact in any serial data recovery logic such as is commonly used with controllers associated with various magnetic media devices, it is desirable to have the capability of decoding Manchester-encoded data accurately and at any one of several data rates. The known prior art serial data recovery logic utilizes single-shot multivibrators to determine the "windows" for the transition intervals in Manchester (also known as FM or biphase) data streams. The single-shot multivibrators are subject to manufacturers' tolerances, drift, and other problems. More importantly, they restrict decoding to a single frequency. In order to avoid the tolerance problems associated with single-shot multivibrators, and to provide automatic accommodation of variable data rates, a totally digital system of serial data recovery is needed.